Energy Institute (EI)guideline ( previously MTD or Marine technology directorate ) provides you with lot of recommedations when the term RHO X V^2 is high where V is velocity and RHO is density. Read this document.

In a nutshell, high velocities excite both low and high modes of vibration.The excitement is in the form of pressure fluctuations and the frequencies are broadband ( this term means that the exciting frequencies are many). In the lower mode range typically <500 Hz the beam based modes of the structure ( pipe) coincide with the acoustic modes of the fluid inside the pipe.In the higher mode range typically >2000 Hz , the shell besed modes of the structure concide with the acoustic modes of the fluid inside the pipe.

Any mode is a standing wave generation due to the fact that all real systems have " ends".When standing waves are generated in a structure we call it a structural mode and when it happens ina fluid we call it an acoustic mode.

This analysis cannot be done using CAESAR II.

Regards